For the rehabilitation of these patients with single sided deafness traditional CROS (Contralateral Routing Of Signal) hearing aids are used today. Such a hearing aid comprises a microphone on the deaf side of the patient and an amplifier with a loudspeaker on the good ear. The sound is then transmitted from the deaf side to the good ear to avoid the head shadow effect which otherwise makes it difficult for a patient to hear anything from the deaf side of the head. One example of this type of hearing aid is described in U.S. Pat. No. 3,809,829.
Another example of a previously known CROS device comprises a powerful traditional hearing aid placed on the deaf ear. In this case, the sound is amplified by the apparatus and picked up in the ear canal and converted into vibrations in the skull bone. The vibrations are then transmitted to the cochlea of the good ear. This type of apparatus is usually named transcranial CROS.
Unfortunately these types of hearing aids for rehabilitation of patients with single sided deafness have significant drawbacks. In the first-mentioned apparatus the hearing in the good ear is reduced, due to the apparatus itself in the ear but also due to the fact that the signal must be transmitted from the microphone on the deaf side to the other side by means of a cable or for instance by means of a FM radio link. A transcranial CROS, on the other side, involves acoustic feed-back problems unless the ear plug is made very tight. Another disadvantage with transcranial CROS devices is the fact that the sound quality is poor in these devices as they often has to be working with full power.
For persons with other types of impaired hearing, for instance a misfunction in the auditory canal or a chronic ear inflammation, there are bone conducting hearing aids on the market today, i.e. bone anchored hearing aids which mechanically transmit the sound information to a person's inner ear via the skull bone. Such a hearing aid is described for instance in U.S. Pat. No. 4,498,461.
In such a bone anchored hearing aid the sound information is mechanically transmitted by means of a vibrator via the skull bone to the inner ear of a patient. The hearing aid device is connected to an implanted titanium screw installed in the bone behind the poor, external ear and the sound is transmitted via the skull bone to the cochlea (inner ear) of this poor ear, i.e. the hearing aid works irrespective of a disease in the middle ear or not. The bone anchoring principle means that the skin is penetrated which makes the vibratory transmission very efficient.
This type of hearing aid device has been a revolution for the rehabilitation of patients with certain other types of impaired hearing. It is very convenient for the patient and almost invisible with normal hair styles. It can easily be connected to the implanted titanium fixture by means of a bayonet coupling or a snap in coupling.
However, these hearing aid devices have substantially been designed for stimulating the inner ear on the same side of the skull where the apparatus is placed, and they have so far not been used for rehabilitation of those patients mentioned above, i.e. patients with single sided deafness. It is an object of the present invention to provide a hearing aid for rehabilitation of the patient category that has been described here, but which hearing aid in contrast to the above-mentioned so-called CROS and BICROS devices is based on the bone conducting principle, i.e. a bone anchored hearing aid in which the vibratory device is mechanically anchored in the skull bone by means of osseointegration.